I have a family that insists on cooking turkey in a bag. I have brined/roasted the turkey the last few years with excellent results, yet they still think the bag method is better. What are the pros/cons of cooking a turkey in a bag vs roasting?

Arnold Ziffel made a comment in the "Should You Cook Your Turkey in Parts?" post about icing the turkey breasts with a ziplock bag before roasting your turkey. This would allow the dark meat and white meat to both come to their optimal temperatures at the same time. Does this really work? Are there any "cons" to doing this?